Photosynthesis is complicated….


Photosynthesis is the basis for most biomass on Earth


  • CO2 + H20 + light —> Sugars + O2


  • Photosynthesis is an energy pathway
    • 2 linked reactions
    • light-dependent reaction + Calvin cycle


  • Everything takes place in the chloroplast
    • light reaction = thylakoid membranes
    • Calvin cycle = stroma (liquid portion)


  • Light reaction - light energy into chemical energy (ATP & NADPH)
  • Calvin cycle - converts CO2 into sugars

Chloroplasts: Where the magic happens

When light hits chlorophyll a few things can happen

When pigments absorb photons an electron gets excited

Photosystems (2) convert light energy to chemical energy

Photosystem = packaging of chlorophyll pigments and proteins in thylakoid membranes

Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle occurs in the stroma

Calvin cycle = uses the chemical energy stored by the light-dependent reactions to form sugars

Calvin cycle is a 3-step process

  • 1. Fixation: CO2 reacts with 5C molecule (RUBP)
    • catalyzed by Rubisco enzyme
    • new 6C split into two acids (3C each)


  • 2. Reduction: ATP and NADPH convert 3C acids into 3C sugars
    • reduction of acids (requires electron carrier)
    • requires energy


  • 3. Regeneration: Some 3C sugars recycled to make RUBP (5C)
    • one 3C sugar leaves to make carbohydrates/sugars
    • recycling step also needs ATP

Making sugars requires several cycles

Three spins needed to remake RUBP and six spins to make glucose (a 6C molecule)

Rubisco - the enzyme that dominates carbon fixation






  • Most abundant enzyme in the world


  • Most CO2 converted into biomass is fixed by Rubisco


  • Large molecule : 16 polypeptides : 8 active sites

Rubisco: the slow, clunky and cheating carbon fixer




  • Only 3-10 reactions per second


  • 20% error rate
    • gets worse at high temperatures



  • Photorespiration - Rubisco cheats on CO2 with O2
    • Rubisco does have a higher affinity for CO2 via natural selection
    • but…. O2 is more prevalent in the atmosphere

Calvin Benson won a Nobel prize, his TA won best practical joke